The invention is related to producing cement clinker.
In known processes for producing cement clinker, raw material fed into a rotary kiln is preheated and partially decarbonated in a multistage cyclone suspension preheater system and a precalciner by using the heat of combustion gases exhausted from the rotary kiln and precalciner. As the combustion gases and raw material mix, lime (CaO) in the raw material and sulfur dioxide (SO2) in the combustion gases react to form calcium sulfite (CaSO3). The calcium sulfite is formed in the preheater and in the main electrostatic precipitator of the stack. The calcium sulfite, in turn, reacts with oxygen in the preheater system to form calcium sulfate (CaSO4), if there is sufficient oxygen. If there is not enough oxygen in the atmosphere at the kiln""s inlet, the calcium sulfate may decompose into lime and sulfur dioxide and leave depositions at the kiln""s inlet. If there is an insufficient excess of oxygen in the rotary kiln, the calcium sulfate may decompose at temperatures of 1200xc2x0 Celsius. Similarly, if there is not enough oxygen in the preheaters, the calcium sulfite may decompose into lime and sulfur dioxide. This decomposition also leads to an increase in sulfur dioxide concentration in the gas in the kiln, which leads to depositions of calcium salts on the shells and walls of the preheater""s cyclones and ducts. The level of deposit formation may be increased when the combustion fuel is a solid fuel high in sulfur (i.e., above 2%), such as petcoke, oil shale, and agricultural or industrial wastes, or a fuel oil high in sulfur content because of the resulting increased sulfur dioxide concentration in the kiln gas. The increased sulfur circulating in the gases causes an increase in the quantity of calcium sulfite. This may result in deposits to a level sufficient to close the kiln inlet, preheater, preheater cyclones, and ducts connecting the cyclones, thereby stopping production. The problem can be alleviated by extracting a fraction of the gas between the rotary kiln and preheater and sending it to a bypass tower. In the bypass tower, the gas is quenched with cooler atmospheric air and a dust rich in sulfur dioxide precipitates out. The desulfurized gas is then directed into the preheater, the result being an overall reduction in the concentration of sulfur dioxide in the gas in the preheater. This solution poses two significant problems: a loss in thermal energy and an environmental issue in disposing of the precipitated dust.
Alternatively, the oxygen can be controlled to ensure an excess oxygen concentration in the kiln and eliminate the need for a bypass tower. However, this potential solution is prone to problems associated with oxygen sensor reliability in a kiln environment, which is further reduced at the kiln inlet where oxygen concentration is even more important. At the inlet, the gas intake for oxygen analyzers can be filled by the dust circulating in the kiln environment. Because current oxygen sensors in the kiln environment may be unreliable, it is not practical to provide continuous control of cement clinker production using an oxygen sensor. To provide excess oxygen by merely increasing the flow of air through the kiln, precalciner, and preheaters may create other problems associated with reduced thermal efficiency and pressure loss.
The invention provides a process having an air intake rate that is regulated based on the quantity of calcium sulfate measured in the cement clinker end product as sulfur or sulfur trioxide (SO3). The air intake rate directly affects the amount of oxygen in the kiln that is available for the reaction converting CaSO3 to CaSO4, and also affects the rate at which they decompose. An increase in the concentration of oxygen to 4.5 to 5.5% increases the temperature at which calcium sulfate decomposes to a temperature greater than the sintering temperature such that CaSO4 becomes a component of the finished product rather than decomposing into gases and leaving deposits in the kiln, preheater, and preheater cyclones. Thus, analysis of the sulfur in the cement clinker end product can be used to control the oxygen concentration in the sintering zone and the reaction zone of the kiln and thereby indirectly control the proportion of sulfur exiting the system as part of the cement clinker.
The air intake to the kiln is mechanically adjusted by increasing or decreasing the speed of a main exhauster that creates a negative pressure that pulls air into and through the kiln, preheater, preheater cyclone""s, and precalciner. The air carries the combusted fuel gases from the kiln and precalciner into the preheater. In the preheater and preheater cyclones, the raw material is preheated and separated from the gases. It also is partially precalcined, i.e., the calcium carbonate in the raw material is partially decomposed into lime and carbonic (CO2) gas. In the precalciner, the raw material is further decarbonated to a level of 90 to 95%. In addition, the gas is desulfurized in the main electrostatic precipitator of the stack and preheater by transfer of the sulfur in the gas to the raw material through the reaction CaO+SO2xe2x86x92CaSO3. Thus, 90 to 95% of the carbonic gas in the raw material is released before the raw material reaches the kiln inlet.
Control of the air intake may be accomplished when using a rotary kiln for producing the cement clinker. The raw material enters the system as a whole at the upper end of the preheater and enters the rotary kiln through an inlet at the kiln""s upstream end, which is connected to the preheater outlet. The inlet also contains a vertical connection to the precalciner through which passes the combustion gases produced by burning fuel at the rotary kiln""s burner. The burner, located at the downstream end of the rotary kiln, produces the heat needed for sintering the raw materials in the kiln. The kiln is inclined to facilitate the flow of material. After the cement clinker passes through the kiln ""s sintering zone, it exits the rotary kiln to the cooler through an outlet adjacent to the burner. The outlet for the cement clinker also serves as an inlet to the rotary kiln for a portion of the air that is blown into the cooler to cool the cement clinker. The air is heated as it cools the cement clinker. The air is blown into the cooler by multiple fans and creates an increase in pressure in the cooler.
The cooling air not flowing into the kiln exits the cooler through two outlets. One outlet directs the air into an electrostatic precipitator to recover fines of the clinker, after which the air is released into the atmosphere. The other outlet directs the air into a dust chamber that returns clinker dust to the cooler and directs the air into the precalciner. A valve on the line between the dust chamber and precalciner regulates the flow of air into the precalciner and affects the proportion of air flowing through these two lines and the kiln. As less air is directed to the precalciner by closing the valve, more air flows through the kiln and electrostatic precipitator of the cooler.
The precalciner decarbonates the raw material using the combustion gases from the rotary kiln and by combusting fuel at a burner in the precalciner. The oxygen for the combustion is supplied as a component of the heated air entering the precalciner from the rotary kiln and through an air inlet connected to the tertiary air duct and located at the base of the precalciner. The raw material feeds into the precalciner from the dust outlet of a cyclone suspension preheater.
The invention permits a more economical use of solid, liquid or gaseous high sulfur fuels in the production of cement clinker in rotary kilns. The invention also permits operating conditions to be maintained so that the sulfur in the fuel is transferred to the cement clinker in the form of CaSO4, which drastically reduces the SO2 concentration in the process and thereby reduces SO2 emissions to a minimum. The invention improves the process of producing cement clinker by permitting the use of fuels containing up to 10% sulfur and reducing the emissions of SO2 and NOx, gases. The 10% sulfur limit is based on using fuels with calorific values of approximately 8,000 kilocalories per kilogram of fuel. The NOx, gas emissions are reduced by creating a reducing atmosphere that uses the O2 of the NOx, in the precalciner. Additionally, if there is enough calcium sulfate in the clinker, no additional gypsum needs to be added to act as a cement setting retarder while grinding the clinker for cement production.
In one general aspect, the invention may produce cement clinker using high sulfur fuels combusted in a kiln through the two burners (i.e., the kiln and the precalciner burners) into which feed material is introduced at an inlet of the preheater connected to the kiln. The feed material is sintered to form sintered material, which is cooled to form cement clinker. The process is controlled by measuring the sulfur concentration in the cooled cement clinker at the cement clinker cooler outlet to control the oxygen concentration at the reaction zone and the sintering zone of the kiln to balance the inlet of sulfur with the outlet in the end product.
Embodiments may include one or more of the following features. For example, the feed material may be precalcined using high sulfur fuels combusted in the precalciner and preheated using the combustion gases from the kiln and precalciner. The oxygen concentration in the precalciner may also be controlled to help use the excess oxygen from the kiln and create a reducing atmosphere in the precalciner to reduce NOx, emissions. The oxygen concentrations in the precalciner and kiln may be varied by adjusting the speed of an exhauster that draws air through the kiln, preheater, and precalciner. A valve positioned in a line between the precalciner and cooler also may be adjusted by a controller to vary the amount of air flowing into the precalciner and affect slightly the oxygen flowing in the kiln.
The oxygen concentration may be controlled to maintain an elevated decomposition temperature of calcium sulfate in the kiln to prevent calcium sulfate decomposition. The fuel used in the burners may contain up to 10% sulfur and the sulfur in the fuel reacts with the CaO of the feed material to form calcium sulfate, which becomes a component of the cement clinker up to weight concentrations of 3%. Because of the calcium sulfate in the cement clinker, no gypsum needs to be added while grinding the cement clinker to produce cement. By controlling the oxygen in the kiln, the circulation of SO2 in the kiln can be reduced to less than 80 kg per hour to eliminate deposits in the kiln, cyclones, and ducts connecting the cyclones.
Embodiments also may include a rotary kiln with a burner to sinter the raw material, a cooler to cool the cement clinker, a sulfur analyzer to measure the sulfur content in the cement clinker and a controller to control the oxygen concentration in the sintering zone and the reaction zone of the kiln based on the measured sulfur content of the cooled cement clinker. The controller may use the measured sulfur content to control the speed of an exhauster to control the oxygen concentration in the kiln. The kiln may be connected to a precalciner that burns high sulfur fuels to precalcine (i.e., decarbonate) the raw material. A cyclone suspension preheater may be connected to the kiln and precalciner to preheat and partially decarbonate the raw material before it enters the kiln. A tertiary air line and valve between the cooler and precalciner may be used to regulate the flow or air to the precalciner. Oxygen sensors may be placed at the kiln inlet and at the gas outlet from the cyclone suspension preheater after the exhauster, and the oxygen sensor and the carbon monoxide sensor may be positioned to monitor oxygen and carbon monoxide, respectively. The carbon monoxide (CO) concentration at the gas outlet of the cyclone suspension preheater may be used by the controller to control the tertiary air line valve. An electrostatic precipitator may be used to filter the air passing from the cooler to the atmosphere.
Other features and advantages will be apparent from the following detailed description, including the drawings, and from the claims.